Computational studies of silica

Abstract

There are three areas of research in this thesis. The �first is concerned with the silica polymorph, tridymite, with simulations carried
out using three computational methods: free energy minimisation,
molecular dynamics and Density Functional Theory. A number
of tridymite structures with di�fferent atomic configurations have
been found in nature. The simulations explore various properties of
these di�fferent forms of tridymite and investigate whether it is possible
to distinguish between them using the three computational
techniques. It was found that the interatomic potential and simulation
technique used, rather than the simulation temperature, were
the main factors a�ffecting the resulting structure. There are a number
of possible explanations for this result: The techniques may not be sensitive enough to deal with an energy landscape as at as in the case of tridymite. Another reason is that the potentials have been
parameterised to distinguish between structures which have reconstructive
transitions (where bonds are broken and formed) and may
not be able to deal with displacive transitions (where only angles
between atoms change) as with tridymite. The �final possible explanation
is that a number of the known structures may be meta-stable
and/or poorly characterised.
For the second research area molecular dynamics simulations using
a rigid ion two body potential were carried out in order to investigate
the properties of silica melts and glasses. A number of different
silica crystals were melted to see whether the melts are all similar or
whether their properties can be di�fferentiated according to the original
crystal structure. At sufficiently high temperatures the starting structure did not a�ffect the properties of the melt. Several properties
of silica melts and glasses were investigated: mean square displacement,
autocorrelation functions, pair distribution functions, the extent
to which silicon and oxygen atoms move together, Arrhenius
plots, coordination number, bond lengths and angles. Investigations
were also carried out as to whether it is possible to use a shell model
to simulate a silica melt. Various properties were calculated and it
was found that agreement with experiment was not as accurate as
when using the rigid ion model.
The third research area is an exploration of the properties of amorphous
silica at elevated pressures and a range of temperatures, using
molecular dynamics with a rigid ion two body potential. Calculations
show that, at low temperatures, the distortion of the tetrahedra
is not recovered upon decompression whereas experimental
results �find complete recovery of the tetrahedra. There is little
available experimental data on the behaviour of silica at both high
pressures and temperatures. Calculations show that at high temperatures
all properties of the initial structure before compression
are recovered.